what causes a baker's cyst

A Baker’s cyst is usually caused by a problem inside the knee joint that leads to excess fluid production. The most common triggers are arthritis and knee injuries, which cause the knee to swell and produce more synovial fluid that can collect behind the knee in the popliteal bursa, forming a cyst. Less often, cysts can be related to meniscal tears or other joint conditions that increase joint fluid. In children, the cysts are more often idiopathic (no clear underlying knee problem) or related to minor knee issues.